1. Field of the Invention
The present invention relates to a divisional type flywheel device as a torsional damper, hereinafter referred to as a torsional damper type flywheel device. More particularly, the present invention relates to a balancing structure for a torsional type flywheel device having asymmetrically arranged or configured members.
2. Description of the Prior Art
Balancing for a so-called divisional type flywheel device, that is, a flywheel device including a drive side flywheel and a driven side flywheel connected by springs is performed after assembly of the flywheel device by adding or removing a correction mass, for example, a hole of small size in either one of the drive side and driven side flywheels. In the prior art divisional type flywheel device, the components are symmetrically disposed relative to an axis of rotation of the flywheel device. Further, any pair of springs located opposite to each other with respect to the axis of rotation are in balance whether or not the springs are compressed and the gravity centers of the springs move around the axis of rotation. Therefore, providing such a small hole can effectively balance the flywheel.
However, if the flywheel device is one of the type which has components that are asymmetric with respect to an axis of rotation of the flywheel device, the following balancing problems are encountered.
First, the total, original imbalance of an asymmetric flywheel device can be as great as several times that of a conventional symmetrically constructed flywheel device. As a result, if an attempt is made to balance the asymmetric flywheel device by means of the prior art correction hole, a far greater amount of holes, in both number and size, than are required for a symmetrical flywheel. Thus, such balancing is undesirable and will deteriorate the characteristics of the flywheel device, for example, the structural strength of the flywheel device.
Second, when a torque acts on the flywheel device to cause relative rotation (torsion) between the drive side and driven side flywheels, the springs also stroke and the gravity centers of the springs move relative to the drive side and driven side flywheels about the axis of rotation. As a result, the direction of the total, initial imbalance of the flywheel device changes to be offset from the direction of the compensating imbalance fixed to either one of the drive side and driven side flywheels, accompanied by an increase in imbalance of the flywheel device. The foregoing will be discussed more fully below with reference to FIG. 10 which illustrates balancing of a flywheel device wherein the spring is asymmetrically arranged with respect to the axis of rotation and a prior art correction hole is provided in only the drive side flywheel
In FIG. 10, when there is no relative rotation between the drive side and driven side flywheels, a total, original imbalance A due to the asymmetric arrangement of the spring and a compensating imbalance A' are in perfect balance. However, when a relative rotation of torsional angle .theta. occurs between the drive side and driven side flywheels, a gravity center of the spring moves about the axis of rotation by a half the torsional angle .theta./2 and moves from g to g'. As a result, the direction of the original imbalance A rotates about the axis of rotation by the angle .theta./2, while the direction of compensating unbalance A' is fixed to the drive side flywheel because the correction hole is formed in the drive side flywheel. Therefore, when a relative rotation occurs, another imbalance B is generated. The imbalance B can be calculated by the following equation: EQU B=2*A* sin (.theta./2)
When the torsional angle .theta. is 30.degree., the imbalance B can be calcurated to be as great as 52% of the original imbalance A. Such a great imbalance is unacceptable.